In a typical filter assembly, a filter element is located within a housing in such a manner that the relevant fluid flows through the filter element and particles are removed therefrom. In one known type of filter element, the filter media comprises a cylindrical construction of pleated material. The filter element can be coreless (i.e., the media's inner radius is self-supporting and received over a support tube integral with the filter housing) or it can include an integral support tube.
A cylindrical pleated filter media is usually made by folding media material into a plurality of longitudinally-extending pleats. The folded media is then shaped into a cylinder with the end pleats being positioned circumferentially adjacent each other. A side seam is then formed between the end pleats to maintain the media in the cylindrical shape. In this shape, the pleats have radially-inner peaks defining an inner diameter, radially-outer peaks defining an outer diameter, and side walls extending therebetween.
The cylindrical filter media can then be mounted over its inner core (if it has one), and end caps can be attached to the opposite axial ends of the filter media. The fluid to be filtered typically passes radially inward through the filter media and then outward through an opening in one of the end caps to an outlet passage in the housing. This radially inward flow direction is usually the most advantageous for efficient filtering, although it could conceivably be more beneficial for the fluid to pass radially outward through the pleated media in certain filtering and/or coalescing situations.
It is important that the pleats of the filter media be able to withstand the pressure of fluid flowing therethrough. If the pleats become deformed (e.g., folded-over and/or bunched against one another), the filtering surface area of the pleats is reduced and the useful life of the filter element is significantly shortened. Accordingly, almost all pleated filter media contain some type of support mechanism for preventing deformation of the pleats. The support mechanism have conventionally been pleatable “endoskeleton” layers incorporated into the pleated media (i.e., resin-reinforced, cellulose-fiber, woven mesh layers) and/or rigid “exoskeleton” structures surrounding the pleated media (i.e., metal cages or rings). Non-rigid exoskeleton support structures, such as spiral wraps and flexible sleeves have also been used in conjunction with pleatable endoskeleton layers.
It is also important that the side seam in the filter media remain structurally sound throughout the filter's life. Any rips or tears in this seam provide bypass flow passages for the fluid thereby compromising the filter's efficiency and perhaps even forfeiting its usefulness. That being said, the side seam construction must also be designed to avoid the sacrifice of precious flow area through the filter media. Depending upon the filtering situation, the optimum side seam construction can be accomplished by sewing, gluing, taping, and/or mechanically clipping the end pleats together.
Of particular relevance to the present invention is a cylindrical filter assembly used to remove impurities in aviation jet fuel handling systems. Such a filter assembly, known in the industry as an aviation fuel microfilter, has a cylindrical filter media constructed to continuously remove dirt of a minimum particle size (about 0.5 μm to about 25.0 μm) from the aviation fuel. This type of high efficiency microfilter element has traditionally included a filter media having an endoskeleton structure with support layers predominantly made up of cellulose fibers.
The Institute of Petroleum has published recommended minimum performance and mechanical specifications for aviation jet fuel microfilters. These specifications include contaminant removal efficiency (e.g., less than 0.15 mg/l particles greater in size than the stated filter rating and maintained up to a differential pressure of 1.5 bar), media migration (e.g., less than 10 fibers per liter), flow rate (e.g., 10 liters/second per meter of effective media length), differential pressure (e.g., at qualification flow rate with clean, dry fuel), and structural strength (e.g., capable of withstanding a differential pressure of 5 bar (72.5 psi) without element rupture or bypassing of seals). The microfilter specifications have been reviewed and accepted by the major aviation fuel companies and microfilter manufactures are expected to satisfy the specifications.